XML 66 R56.htm IDEA: XBRL DOCUMENT v3.10.0.1
Employee Benefits and Share-Based Compensation - Employee Stock Purchase Plan (ESPP) Activity (Details) - 1997 Employee stock purchase plan - Employee stock purchase plan (ESPP) - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Purchases under ESPP (in shares) 289,000 259,000
Exercised (in dollars per share) $ 26.30 $ 25.51
Unrecognized compensation cost $ 4.7  
Weighted average period of compensation cost not yet recognized (in years) 1 year 2 months 12 days